Experimental Physics and
Industrial Control System

Andrew Johnson <[email protected]> · Mon, 28 Mar 2011 10:48:43 -0500

On Monday 28 March 2011 09:57:27 Dirk Zimoch wrote:
>
> If I create a CA client context with
>    ca_context_create (ca_disable_preemptive_callback);
> or, what is the same, with
>    ca_task_initialize();
> in which thread context will then the callback handlers be called? Is it
> in the context of the thread that calls ca_pend_io() or still in the
> context of a separate thread? How was it implemented in 3.13?

In 3.14 the CA client callbacks are *always* made in the context of an 
internal CA thread, and never of the thread that called ca_pend_io() / 
ca_pend_event().  If you use the above calls to initialize the library the 
callbacks are only ever called while the other thread is still executing the 
ca_pend_*() library routine, so you do not have to include mutexes in your 
code.

It occurs to me that there *could* be some circumstances where that's not 
strictly true, if the ca_pend_* thread responds to a signal, but that would be 
fairly unusual and would probably break the rules for signal handlers anyway.

In 3.13 the callbacks were made from the thread that called ca_pend_*(), CA 
was not multi-threaded internally and did all its I/O using the select() OS 
call.  The CAS still uses this approach.

- Andrew
-- 
An error is only a mistake if you don't learn from it.
When you learn something from it, it becomes a lesson.

Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System